The present invention concerns a suspension system for a vehicle, in which a leaf spring is provided to suspend the main body or chassis of the vehicle from a Wheel-bearing axle of the vehicle.
Leaf springs are commonly used in vehicle suspension systems and act to deflect under load changes so as to isolate the main body from vibrations and irregularities occasioned in the course of the vehicle""s travel over a surface. The leaf spring, in an unladen, or partly laden state, assumes a curved shape, the curvature of which increases or decreases under load changes, with concurrent changes in the length of the spring end points.
It is normal practice to provide positive location at one end, usually the front end, of the spring, for example by means of a spring eye and a bush. This arrangement affords a substantially longitudinally fixed disposition for the vehicle""s axle in relation to the main body. The other end, usually the rear end, of the spring is mounted in such a way as to accommodate changes in length of the spring and the shape of the spring at various load conditions is used dynamically to influence the steering and handling of the vehicle.
As an alternative to the use of rubber or polymer mountings, commonly known methods of mounting the relatively unfixed, for example, rear end of the spring, so as to accommodate length changes are a) by the provision of a shackle, or b) by the provision of a cam slider bracket.
In a known suspension system to be described hereinbelow the vehicle main body is suspended from a vehicle axle through a leaf spring having, at its front end, in the intended direction of normal travel of the vehicle, a spring eye and bush which affords a substantially longitudinally fixed disposition of the axle relative to the body. At its rear end, a pivotal shackle for accommodating changes of length of the spring as its curvature changes under varying load conditions, is used. In this known arrangement, forces produced by the angle of the shackle can affect the spring deflection rate in a non-uniform manner at various loads.
In another known suspension system also to be described hereinbelow the arrangement is similar to that described above but the shackle is replaced by a cam slider bracket in which an upper element has a lower surface of non-uniform curvature which bears upon an upper surface of the spring leaf in frictional contact therewith. As the spring leaf angle changes, in the locality of the cam slider bracket, the upper surface of the spring leaf bears upon differing zones of the lower curved surface of the upper element of the bracket, which alters the effective length of the spring leaf. This also changes the spring leaf deflection rate in a non-uniform manner at various loads and the friction between the lower curved surface of the upper element and the spring leaf has an adverse effect on the quality of the ride experienced on the main body of the vehicle. Further, measures to reduce or eliminate wear of the upper element be can made at additional expense in bracket design and manufacture. Even further, the effective length change of the spring leaf can also change the stress distribution along the spring leaf, thus deleteriously affecting its fatigue life. The shape of the lower curved surface of the upper element, and the ineffective length of the spring leaf beyond that element, use up considerable longitudinal installation space which reduces the effective length of the spring leaf available when the longitudinal installation space is limited, which is normally the situation. A modification of this known system includes a cam slider bracket also but at the front end of the spring leaf and a separate radius leaf affording positive longitudinal location for the axle.
In a further known suspension system disclosed in British Patent No. 937027 (Nilsson), the rate of a leaf spring is changed by using a torsion spring to connect at least one of the ends of the leaf spring pivotally to the vehicle frame or chassis, the torsion spring comprising a core element of polygonal cross-section surrounded by a sleeve element of corresponding cross-section Roller-shaped members of rubber or the like are provided between the core and sleeve elements and are deformed upon relative rotational movement therebetween. The leaf spring end is connected rigidly to one of the core and sleeve elements, so that it is tangential to a cylinder surface co-axial to the axis of the torsion spring. Thus, the torsion spring creates a torsional force (torque) as the core and sleeve elements rotate relatively to each other due to the leaf spring and change angle. This resultant torque acts on the leaf spring and, as a consequence, increases the spring rate to stiffen the leaf spring.
Nilsson states that the leaf spring could be preloaded to change the range of deflection of the spring and, also, that the length of the leaf spring could be selected to give the correct geometry capable of avoiding any horizontal deflection. This is a very inefficient way of using a spring rate modifier arrangement. If the spring length is not capable of avoiding a horizontal deflection, then Nilsson suggests that a shackle should be introduced into the system. Thus, the Nilsson suspension system uses torsion springs solely to increase the rates of the leaf springs by stiffening the springs at higher loads and is not capable of reducing the rates of the leaf springs by softening the springs at lower loads.
The invention aims to provide an improved suspension system with mounting for the spring which better accommodates changes in spring length. The inventive system combines the advantage of a low friction of the shackle arrangement but with the simplicity of a smaller number of components and with the ability to vary the deflection rate of the spring under different load conditions, whilst also increasing the load-carrying capacity of the spring at higher loads. All this being achieved with a saving of cost and weight.
The invention also aims to provide a further improved suspension system wherein the spring rate can be altered to create a notional horizontal deflection which is resisted to generate a corresponding horizontal linear force, as opposed to a torsional force (torque), which can act upon the spring to either reduce or increase its rate (softer or stiffer), as required. This softening or stiffening of the spring can take place at different parts of the total defection of the spring within the same suspension system.
Accordingly, one aspect of the invention provides a suspension system for a vehicle, comprising a leaf spring adapted to suspend a main body or chassis of a vehicle from a wheel-bearing axle thereof a rigid arm having one end thereof connected rigidly to the leaf spring at or adjacent an end thereof, and an axis to which the other end of the radial arm is connected pivotally, which is located at a position substantially offset from the neutral axis of the leaf spring and which extends generally normal to the regular working plane of the leaf spring, wherein the position of the offset pivotal axis with respect to the neutral axis of the leaf spring is adapted to vary the rate of the leaf spring, to soften or stiffen the suspension system.
Preferably, the other end of the arm is pivoted substantially freely to the offset pivotal axis. That is to say, the arm is pivotable substantially freely about that axis. Such pivotal axis may be provided by a bush, as will be described hereinbelow, in which case, any resistance to pivotal movement of the arm about the axis would be negligible so that, within the working range of the suspension system, including softening and stiffening of the spring, the other end of the arm would still be pivoted to the axis in a substantially free manner with negligible torsional resistance if any.
Thus, the rate of the leaf spring can be varied or altered by selecting an arm length to create a notional horizontal deflection and by resisting that deflection, create a horizontal linear force, rather than torque, at the axis. This force acts along the arm and on to the spring in either direction, depending on the geometry of the system. By selecting the arm and axis positions it is possible to both soften and stiffen the effective rate of the spring as required, by effectively maintaining the spring in a different deflection position than would be the case if the spring were to be deflected in a freely installed state. This softening and stiffening can take place at different parts of the total deflection within the same system.
The offset pivotal axis may be adapted to take up any notional displacement, preferably a substantially linear notional displacement, thereof arising, in operation, from deflections of the leaf spring outside predefined spring loading and deflection ranges. Thus, the offset pivotal axis may be flexible for accommodating said take-up, preferably comprising a bush which may define internal voids therein for accommodating said take-up which, as mentioned above, can be substantially linear.
By this modification, there is introduced a controlled horizontal rate of deflection, rather than a solid resistance thereto, thereby providing for both control of the notional horizontal force and its effect upon leaf spring deflection, thus changing its rate and internal stress. By reducing initial horizontal forces to zero during initial deflections, using voids in, say, a bush and selecting the geometry of the system, a range of defections can be achieved, which have no effective horizontal forces, thus allowing the spring to deflect freely. Also, the voids allow for practical installation, by allowing for installation tolerances.
The position of the offset pivotal axis with respect to the neutral axis of the leaf spring in the rest condition of the system can be located such that, in operation, deflections of the leaf spring over predefined spring loading and deflection ranges produce negligible, if any, displacement of the offset pivotal axis from its rest position, in which case, the offset pivotal axis may be adapted to take up any notional displacement thereof arising, in operation, from deflection of the leaf spring outside said predefined spring loading and deflection ranges. Again, the offset pivotal axis may be flexible for accommodating said take-up, comprising, say, a bush defining, for example, internal voids therein.
In a preferred embodiment of the inventive suspension system, the position of the offset pivotal axis with respect to the neutral axis of the leaf spring and the pivotal connection of the other end of the arm to the axis are, in operation, adapted to provide operational forces resulting from deflections of the leaf spring outside defined spring loading and deflection ranges, which forces create, in a relatively lightly loaded state of the suspension system, a softening effect of the leaf spring and, in a relatively heavily loaded state of the suspension system, a stiffening effect of the leaf spring.
The leaf spring may be mounted to a transverse anti-roll device to which the vehicle body or chase is mounted, with the offset pivotal axis being in concentric relationship to the anti-roll device. In one embodiment, the leaf spring is mounted to the traverse anti-roll device by means of a first bracket, with the mounting of the vehicle body or chassis to the anti-roll device by means of a second bracket which has a portion housing the offset pivotal axis in concentric relationship to the anti-roll device.
Alternatively, the leaf spring may be mounted to a transverse anti-roll device to which the body chassis is mounted, with the offset pivotal axis being in non-concentric relationship to the anti-roll, device, in which case, the leaf spring could be mounted to the anti-roll device by means of a first bracket, with the mounting of the vehicle body or chassis to the anti-roll device being by means of a second bracket which has a portion housing the offset pivotal axis in non-concentric relationship to the anti-roll device.
The anti-roll device may be a torsion bar or tube and the radial arm and associated offset pivotal axis can be located at or adjacent either end of the leaf spring or at or adjacent both ends of the leaf spring.
In another embodiment, the leaf spring is auxiliary to a main leaf spring which mounts a wheel axle of the vehicle. Also, the invention provides a vehicle incorporating a suspension system according to the one aspect of the invention defined above or any modifications thereof.
Throughout this specification, the following terms, which are well known and used extensively throughout the vehicle suspension industry, are used and defined as follows:
xe2x80x9cregular working planexe2x80x9dxe2x80x94that plane, sometimes referred to as the xe2x80x9cbending planexe2x80x9d, in which a spring leaf in deflected (bent) under normal operating conditions, namely, one which is generally vertical and extends in the fore-and-aft direction of the vehicle;
xe2x80x9cnormal to the regular working planexe2x80x9dxe2x80x94a perpendicular to the regular working or bending plane of the spring leaf,
xe2x80x9cneutral axisxe2x80x9dxe2x80x94to all intents and purposes, a basic spring leaf is considered to be a regularly-sectioned beam which deflects under a bending load to create a tension stress in the upper surface of the beam (spring leaf) and a compression stress in the lower surface of the beam (spring leaf), the xe2x80x9cneutral axisxe2x80x9d being that internal layer of the beam (spring leaf) which extends longitudinally and approximately centrally of the section of the beam (spring leaf), which undergoes zero (neutral) stress and about which the beam (spring leaf) deflects (bends) to absorb the energy caused by such deflection (bending);
xe2x80x9cspring ratexe2x80x9dxe2x80x94deflection of a spring leaf under a specific change of load at that particular loading, which is usually measured in force per unit length of deflection, for example, a spring leaf having a spring rate of 1000 pounds per inch will deflect under a load change of 1000 pounds by one inch; and
xe2x80x9cnotional displacementxe2x80x9dxe2x80x94an imaginary movement (displacement) of a component of a vehicle suspension which does not actually take place due to a restraint or resistance placed upon the component by at least one other component of the suspension but which would otherwise take place in the absence of such a restraint or resistance. For example, and in relation to the offset pivotal axis of the inventive suspension system, xe2x80x9cnotional displacementxe2x80x9d of that axis is a movement (displacement) thereof which would otherwise, but does not, arise from deflections of the spring leaf outside defined spring loading and deflection ranges. In certain circumstances, however, the term xe2x80x9cnotional displacementxe2x80x9d may be used alternatively to embrace a displacement (movement) which is so small as to be considered negligible in the context of the inventive suspension system and, thus, effectively being an imaginary movement (displacement) as defined above.
Preferably, the rigid arm is a radial one having one end connected rigidly to the leaf spring at or adjacent an end thereof and its other end connected pivotally to said axis.
As discussed above, the pivotal axis is preferably in the form of a bush provided either at the rear end or the front end of the leaf spring, or there may be such a bush or other pivotal axis at both ends, thereof. The centre of the or each bush affords an axis substantially offset from the neutral (bending) axis of the leaf spring, whose rest position in relation to the adjacent end of the spring may be selected such that no, or substantially no, actual or notional displacement of the offset axis from its rest position will occur for deflections of the spring within predefined spring loading and deflection ranges. However, for deflections of the spring outside the predefined ranges, any notional displacement of the offset pivotal axis will be taken up by the arrangement or construction of the bush or other means, for example, by providing the bush with internal voids.
Preferably, the positioning of the pivotal axis with respect to the region of the end of the leaf spring to which it is connected by the rigid arm is such that (a) in a relatively lightly loaded state of the suspension system, operational or working forces create a softening effect of the spring and (b) in a relatively heavily loaded state of the suspension system, operational or working forces will create a stiffening effect of the spring, such operational or working forces arising from deflection of the spring outside predefined spring loading and deflection ranges.
Again, the pivotal axis may be in the form of a bush, although other suitable forms of pivotal axis means may be employed.
The arrangement of the or each bush or other pivotal axis in relation to the adjacent end of the spring is such that any substantially longitudinally-directed displacement of the bush or other pivotal axis means, arising from a change in curvature and effective length of the spring, is mostly compensated by an opposite longitudinally-directed displacement of the bush or other pivotal axis means arising from a change of the local spring angle. Thus, for spring deflections within predefined spring loading and deflection ranges the bush or other pivotal axis means will experience no, or substantially no, actual displacement; whereas, for deflections outside these defined ranges, any notional displacement may be taken up by the bush or other pivotal axis means.
Another aspect of the invention resides in a method of varying the rate of a leaf spring in a vehicle suspension system comprising a main vehicle body or chassis suspended from a wheel-bearing axle of the vehicle by the leaf spring, the method including connecting one end of a rigid arm to the leaf spring at or adjacent an end thereof, converting the other end of the rigid arm pivotally, preferably substantially freely, to an axis extending generally normal to the regular working plane of the leaf spring, positioning the axis substantially offset from the neutral axis of the leaf spring and taking up any notional displacement, such as substantially negligible linear displacement, of the axis arising, in operation, from deflections of the leaf spring outside predefined spring loading and deflection ranges.
The invention will better be appreciated from the following description of embodiments thereof given by way of example and with reference to the accompanying drawings in which: